Some cache memories may be structured as “sectored” cache, in which multiple cache lines for example, 64 cache lines are grouped to form a “superline” which is identified by a single tag such that all the cache lines within that superline are identified by the single tag. In a sector write-back cache, a complete superline must be evicted if it is chosen as a replacement (i.e., victim) candidate.
A sectored cache assumes spatial and temporal locality (i.e. accessing one sector (cache line) within a superline means that most of the superline is used). However, studies show that there are exceptions to this rule, and the exceptions tend to be extreme. For example, if a superline is not fully used, there is a high probability that only a small portion of it is used for a relatively short period of time. These sparsely used superlines may occupy valuable cache space without much benefit.